pigments could have such an effect on your preservative system - but they do! As with the inactivation of preservatives by non-ionic ingredients, this problem is one for the parabens. If you make mineral make-up, this really relates to you! (Click here to read the entire study.)
Inorganic pigments are formed from compounds of transition elements like iron, chromium, and so on. They include the ultramarines, iron oxides, chromiums, and manganese pigments. This also includes titanium dioxide and talc.
The worst culprit is the ultramarine blue, followed by talc, titanium dioxide, red iron oxide, and yellow iron oxide. Kaolin and silica can have a huge impact on your paraben effectiveness as well, but they're lower down the list.
If you've ever tried to make your own sunscreen (bad idea!), made a soothing cream with titanium dioxide, or created a lovely clay mask, this should give you pause for thought!
How can we overcome these problems? Add a little EDTA at about 0.2% to help with chelating the metals or use another preservative when you're working with these ingredients. And consider your pH - high levels of the ultramarines can take your product from acidic to neutral or alkaline, and we know the parabens like to have pH levels of 8.0 or lower, so you can adjust the pH of your product using citric acid or vinegar.
Join me tomorrow for more information on inactivation of our preservatives by yet another ingredient - cellulose derivatives!